U.S. patent application number 10/058557 was filed with the patent office on 2003-07-31 for semiconductor wafer having a thin die and tethers and methods of making the same.
Invention is credited to Chen, Shiuh-Hui Steven, Garza, Raymond, Ross, Carl, Turalski, Stefan.
Application Number | 20030141570 10/058557 |
Document ID | / |
Family ID | 27609616 |
Filed Date | 2003-07-31 |
United States Patent
Application |
20030141570 |
Kind Code |
A1 |
Chen, Shiuh-Hui Steven ; et
al. |
July 31, 2003 |
SEMICONDUCTOR WAFER HAVING A THIN DIE AND TETHERS AND METHODS OF
MAKING THE SAME
Abstract
A semiconductor wafer (70) that includes a support body (72), at
least one thin die (20, 60), and a plurality of tethers (78, 178).
The support body (72) is made of a semiconductor material. The thin
die (20, 60) has a circuit (21) formed thereon and has an outer
perimeter (74) defined by an open trench (76). The open trench (76)
separates the thin die (20, 60) from the support body (72). The
tethers (78, 178) extend across the open trench (76) and between
the support body (72) and the thin die (20, 60). A method of making
a thin die (20, 60) on a wafer (70) where the wafer (70) has a
support body (72), a topside (82) and a backside (90). A circuit
(21) is formed on the topside (82) of the wafer (70). The method
may include the steps of: forming a cavity (88) on the backside
(90) of the wafer (70) beneath the circuit (21) that defines a
first layer (92) that includes the circuit (21); forming a trench
(76) around the circuit (21) on the topside (82) of the wafer (70)
that defines an outer perimeter (74) of the thin die (20, 60);
forming a plurality of tethers (78, 178) that extend across the
trench (76) and between the wafer support body (72) and the thin
die (20, 60); and removing a portion of the first layer (92) to
define the bottom surface (75) of the thin die (20, 60).
Inventors: |
Chen, Shiuh-Hui Steven;
(Lake Zurich, IL) ; Garza, Raymond; (Huntley,
IL) ; Ross, Carl; (Mundelein, IL) ; Turalski,
Stefan; (Chicago, IL) |
Correspondence
Address: |
MOTOROLA, INC.
1303 EAST ALGONQUIN ROAD
IL01/3RD
SCHAUMBURG
IL
60196
|
Family ID: |
27609616 |
Appl. No.: |
10/058557 |
Filed: |
January 28, 2002 |
Current U.S.
Class: |
257/618 ;
438/106 |
Current CPC
Class: |
Y10S 438/977 20130101;
H01L 21/6838 20130101; H01L 21/67132 20130101; G01L 9/0054
20130101; H01L 21/68728 20130101 |
Class at
Publication: |
257/618 ;
438/106 |
International
Class: |
H01L 021/44; H01L
021/48 |
Claims
What is claimed is:
1. A semiconductor wafer comprising: a support body made of a
semiconductor material; at least one thin die having a circuit
formed thereon, the thin die having an outer perimeter defined by
an open trench, the open trench separating the thin die from the
support body; and a plurality of tethers extending across the open
trench and between the support body and the at least one thin
die.
2. The semiconductor wafer of claim 1 wherein the support body has
a first thickness and the at least one thin die has a second
thickness, the second thickness being substantially less than the
first thickness.
3. The semiconductor wafer of claim 1 wherein at least one of the
plurality of tethers is substantially triangular in shape.
4. The semiconductor wafer of claim 3 wherein the at least one
substantially triangular tether has a base and a tip, the base of
the tether being attached to the support body of the wafer and the
tip of the tether extending across the trench and attached to the
at least one thin die.
5. The semiconductor wafer of claim 1 wherein at least one of the
plurality of tethers has a portion that extends across the open
trench, the portion extending across the open trench having its
smallest width adjacent to the outer perimeter of the at least one
thin die.
6. The semiconductor wafer of claim 1 wherein at least one of the
plurality of tethers has a portion that extends across the open
trench, the portion extending across the open trench having at
least a portion of a groove.
7. The semiconductor wafer of claim 1 wherein at least one of the
plurality of tethers has a portion that extends across the open
trench, the portion extending across the open trench having at
least a portion of a hole.
8. The semiconductor wafer of claim 1 wherein the circuit of the
die is adapted for a pressure sensor.
9. The semiconductor wafer of claim 1 wherein the plurality of
tethers are made of a polyimide material.
10. A wafer comprising: a support body made of a semiconductor
material; at least one thin semiconductor die having a circuit
formed thereon, the thin semiconductor die having an outer
perimeter defined by an open trench, the open trench separating the
thin semiconductor die from the support body; and a means for
attaching the outer perimeter of the at least one thin
semiconductor die to the support body across the open trench.
11. The wafer of claim 10 wherein the means for attaching the outer
perimeter of the at least one thin semiconductor die to the support
body across the open trench includes a plurality of tethers.
12. The wafer of claim 11 wherein the tethers are made of a
polyimide material.
13. A method of making a thin die on a wafer, the wafer having a
support body, a topside and a backside, a circuit formed on the
topside of the wafer, the method comprising the steps of: forming a
cavity on the backside of the wafer beneath the circuit that
defines a first layer, the first layer includes the circuit;
forming a trench around the circuit on the topside of the wafer
that defines an outer perimeter of the thin die; forming a
plurality of tethers that extend across the trench and between the
wafer support body and the thin die; and removing a portion of the
first layer to define the bottom surface of the thin die.
14. The method of claim 13 wherein the step of forming the cavity
on the backside of the wafer includes wet etching the backside of
the wafer.
15. The method of claim 13 wherein the step of forming the trench
on the topside of the wafer includes reactive ion etching to form
the trench.
16. The method of claim 13 wherein the tethers are made of a
polyimide material.
17. The method of claim 13 wherein the step of forming of the
tethers includes patterning the tethers so they are substantially
triangular.
18. The method of claim 13 wherein the step of removing the portion
of the first layer includes reactive ion etching the first
layer.
19. A method of forming tethers on a wafer to retain a thin die to
a support body of the wafer, the wafer having a topside and a
backside, the thin die positioned adjacent to the topside of the
wafer, the method comprising the steps of: forming a cavity on the
backside of the wafer beneath the thin die that defines a first
layer, the first layer includes the thin die; forming a trench
around the thin die on the topside of the wafer that defines an
outer perimeter of the thin die and extends between the thin die
and the support body; patterning a polyimide material on the top
surface of the wafer to define the tethers, the tethers extending
across the trench and between the thin die and the support body;
and removing a portion of the first layer to expose the trench such
that the tethers provide the attachment between the thin die and
the support body.
20. The method of claim 19 wherein the step of patterning a
polyimide material on the top surface of the wafer defines the
tethers in a substantially triangular shape.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is related to the following co-pending and
commonly assigned patent application, which is hereby incorporated
by reference herein: application Ser. No. ______, entitled "METHOD
OF SEPARATING AND HANDLING A THIN SEMICONDUCTOR DIE ON A WAFER,"
filed on same date herewith, by Shiuh-Hui Steven Chen, Cheryl
Field, Didier R. Lefebvre, and Joe Pin Wang, attorney's docket
number AP01985.
FIELD OF THE INVENTION
[0002] This invention in general relates to the making and handling
of a very thin semiconductor die and, more particularly, to an
improved device and procedure for fabricating, separating and
transporting very thin dice for better throughput and yield.
BACKGROUND OF THE INVENTION
[0003] As technology progresses, integrated circuits are being
formed on smaller and thinner semiconductor dice for a variety of
applications. Relatively thin integrated circuits (ICs) or
semiconductor dice, also known as "ultra-thin" or "super-thin" ICs
or dice, are used in applications such as smart cards, smart
labels, sensors, and actuators. A thin die for sensors is described
in pending application Ser. No. 09/629,270, filed on Jul. 31, 2000,
entitled "Strain Gauge" by Shiuh-Hui Steven Chen, et al.,
incorporated herein by reference in its entirety. There, a
relatively thin semiconductor die with piezo-resistors act to
measure the pressure of fluids in vehicles. The thin semiconductor
die is bonded to a stainless steel port in order to measure
diaphragm deformation.
[0004] For smart card applications, the thickness of the die may be
as low as 100 micrometers (.mu.m). In the future, it is anticipated
that an even smaller thickness will be necessary. For sensors, a
thin die may have a thickness of between 5 and 50 .mu.m as
described in application Ser. No. 09/629,270.
[0005] When making and handling a very thin semiconductor die, care
must be taken not to fracture or otherwise damage the die.
Currently, a need exists for improved methods and procedures to
fabricate, separate, and transport a thin die for high volume
applications where automated techniques are required to produce
high throughput and acceptable yields.
[0006] It is known to separate and handle integrated circuits on
thin semiconductor die by mechanical grinding, chemical etching and
dry etching with the assistance of adhesive or UV related release
tapes and carrier wafers. Some of the approaches taken in the
electronics industry to separate thin wafers into dice and handle
thin dice include dicing by cutting and dicing by thinning. In
dicing by cutting, a dicing tape is mounted on frames. The wafers
are mounted to the dicing tape, backside down. Dicing is carried
out by sawing, laser cutting, dry etch, etc. After cutting, the
dice are separated on the dicing tape and sent to the assembly line
on a wafer frame for pick and place. The thin die is then ejected
from the backside of the tape with the help of an ejector pin and
picked by a vacuum tip. An example of this process flow is
described in Muller et al., "Smart Card Assembly Requires Advanced
Pre-Assembly Methods," SEMICONDUCTOR INTERNATIONAL (July 2000)
191.
[0007] In dicing by thinning, trenches are etched or sawed on the
topside of a device wafer. Laminating tapes are then placed on a
carrier wafer for mounting the carrier wafer to the topside of the
device wafer. The bottom side of the device wafer is then thinned
until the topside trenches are opened from the bottom side. A
second carrier wafer is then mounted to the bottom side of the
device wafer by a high-temperature release tape. The first carrier
wafer is removed and then the thin dice can be removed by locally
heating a vacuum-picking tool. An example of this process flow
requiring multiple carrier wafers and tape transfers is described
in C. Landesberger et al., "New Process Scheme for Wafer Thinning
and Stress-Free Separation of Ultra Thin ICs," published at
MICROSYSTEMS TECHNOLOGIES, MESAGO, Dusseldorf, Germany (2001).
[0008] Alternatively, it has been known to saw or cut a carrier
wafer into carrier chips, each of them carrying a thin die. In this
case, the carrier chip is removed after die bonding by thermal
release of the adhesive tape. An example of this process flow is
described in Pinel et al., "Mechanical Lapping, Handling and
Transfer of Ultra-Thin Wafers," JOURNAL OF MICROMECHANICS AND
MICROENGINEERING, Vol. 8, No. 4 (1998) 338.
[0009] Conventional procedures have been met with a varying degree
of success. The combination of carrier transfers and tape transfers
necessitate multiple steps with long cycle times and yield loss.
Moreover, the use of heat release and other tapes may exhibit
unacceptable residual adhesion. Further, when used in combination
with an ejector pin, the edges may not delaminate from the tape due
to the lack of flexural rigidity of the thin die and due to the
die's small size in the in-plane directions. The small size of the
die may also limit the net suction force that could be exerted by
the vacuum tip to overcome residual tape adhesion. With regard to
conventional dicing and wafer sawing methods, these steps often
result in damage to the thin die that causes device failure or
performance degradation. Conventional ejector pins may exert
excessive stress that damages the thin die, also causing cracking
and device failure. Carrier transfer or tape transfer may lead to
die contamination on both sides of the die. Multiple transfers by
wafer carriers typically lead to lower yield due to increased
handling and contamination. In the case of a very thin die for
sensor applications, organic adhesive may leave residue on the die
surface, causing poor bonding with the surface being measured.
[0010] It is, therefore, desirable to provide an improved device
and method of fabricating, separating and handling very thin dice
to overcome most, if not all, of the preceding problems.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is an enlarged view of one embodiment of a thin
semiconductor-sensing die with an array of strain gauges positioned
in a Wheatstone bridge arrangement;
[0012] FIG. 2 is an enlarged view of another embodiment of a thin
semiconductor-sensing die with a single transverse strain
gauge.
[0013] FIG. 3 is a side view of a thin semiconductor-sensing die
mounted on a diaphragm.
[0014] FIG. 4 is an exploded partial top view of one embodiment of
a wafer having a thin die with tethers.
[0015] FIG. 5 is an exploded partial top view of another embodiment
of a wafer having a thin die with tethers.
[0016] FIGS. 6A-6D are cross-sectional views of a process to form
tethers that extend between a support body and a thin die of a
wafer.
[0017] FIG. 7 is a top view of one embodiment of a rigid backing
for a wafer of the present invention.
[0018] FIG. 8 is a cross-sectional view of one embodiment of a die
handler for pick and place operations.
[0019] FIGS. 9A-9D are side views of one procedure of the present
invention for separating and extracting a thin die from a
wafer.
[0020] FIGS. 10A-10D are side views of one procedure of the present
invention for transporting and installing a thin die on a
surface.
[0021] While the invention is susceptible to various modifications
and alternative forms, specific embodiments have been shown by way
of example in the drawings and will be described in detail herein.
However, it should be understood that the invention is not intended
to be limited to the particular forms disclosed. Rather, the
invention is to cover all modifications, equivalents and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims.
DETAILED DESCRIPTION
[0022] What is described is an improved device and method of making
and handling a thin semiconductor die including the fabrication,
separation and transfer of such die. For purposes of illustration
and description, an example of an application of a thin
semiconductor die is described below in the context of a thin
semiconductor-sensing die for sensing the pressure of fluids in a
vehicle. However, the present invention is not limited to the
making and handling of dice for sensors but may also apply to other
thin dice applications such as smart cards, smart labels,
actuators, and multi-thin wafer designs. One of ordinary skill in
the art having the benefit of this disclosure will realize that the
devices and procedures described herein for the making and handling
of thin dice could be used in other semiconductor applications.
[0023] To this end, in one embodiment there is a semiconductor
wafer that includes a support body, at least one thin die, and a
plurality of tethers. The support body is made of a semiconductor
material. The thin die has a circuit formed thereon and has an
outer perimeter defined by an open trench. The open trench
separates the thin die from the support body. The tethers extend
across the open trench and between the support body and the thin
die.
[0024] The support body may have a first thickness and the thin die
may have a second thickness, wherein the second thickness is
substantially less than the first thickness. In one embodiment, the
tethers may be substantially triangular in shape but other shapes
may be used such as substantially rectangular, elliptical,
semi-circular, or square. It is preferred that the portion of the
tether that extends across the open trench has its smallest width
adjacent to the outer perimeter of the thin die. This provides a
cohesive failure point (or break point) of the tether to be along
the outer perimeter of the thin die to prevent any residual
overhangs during subsequent pick and place operations. Moreover,
this cohesive failure point should mean that the tether itself
breaks rather than being peeled from the thin die. The tether may
also be patterned such that at least a portion of a groove or hole
extends into the portion of the tether that goes across the open
trench. The tethers may be made of a material such as a polyimide
although other materials may be used to support the thin die to the
support body.
[0025] In another embodiment, there is a method of making a thin
die on a wafer where the wafer has a support body, a topside and a
backside. A circuit is formed on the topside of the wafer. The
method may include the steps of: forming a cavity on the backside
of the wafer beneath the circuit that defines a first layer that
includes the circuit; forming a trench around the circuit on the
topside of the wafer that defines an outer perimeter of the thin
die; forming a plurality of tethers that extend across the trench
and between the wafer support body and the thin die; and removing a
portion of the first layer to define the bottom surface of the thin
die.
[0026] In another embodiment, there is a method of forming tethers
on a wafer to retain a thin die to a support body of the wafer. The
wafer has a topside and a backside. The thin die is positioned
adjacent to the topside of the wafer. The method may include the
steps of: forming a cavity on the backside of the wafer beneath the
thin die that defines a first layer that includes the thin die;
forming a trench around the thin die on the topside of the wafer
that defines an outer perimeter of the thin die and extends between
the thin die and the support body; patterning a polyimide material
on the top surface of the wafer to define the tethers, the tethers
extending across the trench and between the thin die and the
support body; and removing a portion of the first layer to expose
the trench such that the tethers provide the attachment between the
thin die and the support body.
[0027] Now, turning to the drawings, an example use of thin
semiconductor dice will be explained and then a wafer with a thin
die and tethers along with a method of separating and handling the
thin die will be explained.
[0028] Example use of Thin Semiconductor Dice
[0029] For purposes of illustration and description, a thin
semiconductor die will be explained in the context of sensors for
measuring the pressure of fluids in a vehicle. Such a thin die for
sensors is described in detail in pending application Ser. No.
09/629,270, filed on Jul. 31, 2000, entitled "Strain Gauge" by
Shiuh-Hui Steven Chen, et al., incorporated herein by reference in
its entirety.
[0030] An example of a thin semiconductor die is shown in FIG. 1.
The thin semiconductor die 20 in this example is a die for a sensor
that measures the pressure of fluids in vehicles and may range from
5 to 50 micrometers (.mu.m) thick. The die 20 has sufficient
structural strength and integrity to support one or more strain
gauges 22, 24, 26, 28. In this case, the die 20 is generally square
and has a geometric center 30. Metal bond pads 32, 34, 36, 38 are
positioned in proximity and adjacent to the corners of the die 20.
A set, series, or array of silicon oxide openings providing
electrical contacts 42, 44, 46, 48 are disposed and securely
positioned underneath the pads 32, 34, 36, 38. The die 20 has
semiconductors 52, 54, 56, 58 (such as P+ doped silicon-containing
interconnects) that provide interconnects between the strain gauges
22, 24, 26, 28 and the electrical contacts 42, 44, 46, 48.
[0031] The die 20 illustrated in FIG. 1 has strain gauges 22, 24,
26, 28 with interconnected resistors positioned in a Wheatstone
bridge arrangement. The gauges 22, 24, 26, 28 measure strain in
response to and induced by pressure of a fluid, such as fluid in a
vehicle. Accordingly, referring to FIG. 3, the thin
semiconductor-sensing die 20 may be mounted to a fluid responsive
diaphragm 40. The thin semiconductor-sensing die 20 and fluid
responsive diaphragm 40, and how it may interconnect with a fluid
housing, is further described in application Ser. No. 09/629,270,
filed on Jul. 31, 2000, entitled "Strain Gauge" by Shiuh-Hui Steven
Chen, et al. In sum, the fluid responsive diaphragm 40 can be
positioned to contact the sensed fluid in the vehicle. These fluid
responsive diaphragms are preferably made of a corrosion-resistant
material (such as stainless steel) that will not readily corrode in
the fluid being sensed.
[0032] A symmetrical pressure-conductive coupling 50 can be
provided to connect the semiconductor die to the diaphragm. The
coupling 50 may include a corrosive-resistant pressure-conductive
electrically insulating material to conduct and transmit the sensed
pressure from the diaphragm to the thin semiconductor-sensing die
20. A suitable coupling 50 is made of fused glass frit and
screen-printed on the diaphragm 40. Glass frit is useful because it
electrically isolates and prevents shorts from the metal diaphragm
40.
[0033] Another embodiment of a thin semiconductor-sensing die 60 is
shown in FIG. 2. The thin semiconductor-sensing die 60 as shown in
FIG. 2 is structurally and functionally similar to the one shown in
FIG. 1 but has a single transverse strain gauge 62. The single
transverse strain gauge 62 is registered and positioned in
alignment with the geometrical center 64 of the die 60. This helps
minimize electrical effects of thermal stress on the transverse
strain gauge during measuring and operation of the vehicle. Here,
the transverse strain gauge can include a single four contact
resistor element oriented to maximize response to pressure induced
stresses through shear stress effects. A further description of the
thin semiconductor sensing die 60 and strain gauge 62 are provided
in application Ser. No. 09/629,270, filed on Jul. 31, 2000,
entitled "Strain Gauge" by Shiuh-Hui Steven Chen, et al.,
incorporated herein by reference in its entirety.
[0034] As with other thin semiconductor dice, there is a continuing
need to improve the separation and handling of a thin die after
fabricating the integrated circuit thereon. In particular, there is
an ongoing need to increase throughput in a low cost automated
environment and to provide better yields in such an
environment.
[0035] Wafer with Thin Die and Tethers
[0036] A new device and process has been developed to assist in
separating a thin semiconductor die from a wafer. An integrated
circuit 21 is initially formed on a standard wafer. Further
fabrication processes to help in subsequent separation of a die
(that includes the circuit) from the wafer are illustrated in the
top views of FIGS. 4 and 5 and in the cross-sectional views of
FIGS. 6A-6D. The device and process includes the formation of thin
tethers around the perimeter of the die. This allows for easier
separation of the die from the wafer in subsequent processes.
Again, for purposes of illustration, the description and figures
are shown in the context of the thin semiconductor die 20 described
above in FIG. 1. One of ordinary skill in the art with the benefit
of this disclosure will recognize, however, that the present
invention applies to other thin die applications.
[0037] Referring to FIG. 4, an exploded portion of a semiconductor
wafer 70 is shown having a support body 72 made of a semiconductor
material and at least one thin semiconductor die 20. The thin die
20 has an integrated circuit (generally referenced as 21) formed
thereon. The thin die 20 also has an outer perimeter 74 defined by
an open trench 76. The open trench 76 separates the thin die 20
from the support body 72 of the wafer 70. A plurality of support
tethers 78 extend across the open trench 76 and between the support
body 72 and the thin die 20. In one embodiment, as will be seen in
the cross-sectional views of FIGS. 6A-6D, the thickness of the thin
die 20 is substantially less than the thickness of the support body
72.
[0038] The tethers 78 may have a variety of geometric patterns and
sizes. In one embodiment, as shown in FIG. 4, the tethers 78 may be
substantially triangular. Here, the substantially triangular
tethers 78 have a base 80 that is formed on the topside 82 of the
wafer 70 and a tip 82 that extends across the open trench 76 and
onto the die 20. The tip 82 of the tether 78 may be patterned so
that it is partially cutoff to limit the portion of the tether 78
extending on the die 20. The tether 78 should, however, extend
sufficiently onto the die 20 to allow the die 20 to be retained to
the wafer support body 72. This attachment should be sufficient to
withstand normal shipping and handling requirements for a standard
wafer. In one embodiment, for a die 20 having a thickness of about
15 .mu.m, each of the tethers extend at least 10 .mu.m over the
outer perimeter 74 of the die 20.
[0039] In another embodiment, as shown in FIG. 5, a tether 178 is
also substantially triangular but is patterned with grooves 184.
The substantially triangular tethers 178 have a base 180 that is
formed on the topside 82 of the wafer 70 and a tip 182 that extends
across the open trench 76 and onto the die 20. The grooves 184 are
at least partially formed in the portion of the tether 178 that
extends over the trench 76. The grooves 184 define a neck 186 that
extends between the two grooves 184. The benefit of including
grooves 84 in the formation of the tethers 178 is that they allow
for better separation of the die 20 from the wafer 70 during pick
and place operations. Although the specific width of the neck 186
is application specific, in one embodiment for a thin die 20 having
a thickness of about 15 .mu.m, the width of the neck 186 may have
ranges between 10 and 40 .mu.m. What is important is that a
cohesive failure point (or break point) of the tethers 178 be along
the edge of the semiconductor die and such that the tether itself
breaks rather than being peeled from the thin die during pick and
place operations. This break point should be sufficiently wide to
withstand normal shipping and handling requirements for a standard
wafer--yet be sufficiently thin to break along the outer perimeter
of the die 20 during pick and place operations. As shown in FIGS. 4
and 5, in a preferred embodiment, the portion of the tethers 78,
178 extending across the open groove 76 has its smallest width
adjacent to the outer perimeter 74 of the die 20. This permits the
break point to be right at the outer perimeter 74 to limit any
overhang of the tether that may result after die separation.
[0040] Although FIGS. 4 and 5 show substantially triangular
tethers, the tethers may also be of other exotic geometric shapes
such as substantially rectangular, elliptical, semi-circular, or
square. Additionally, to provide better break points above trench
76, the grooves 184 in the tethers 178 may be replaced with holes
or slots in the tethers 78 along the trench 76. Depending on the
geometric shape of the tether, the addition of grooves, holes or
slots may enable the tethers to have a better cohesive failure
point along the outer perimeter 74 of the semiconductor die 20.
[0041] A process for making or forming the tethers 78, 178 for a
thin die 20 on a wafer 70 will now be explained. Referring now to
FIG. 6A, after forming the circuit on the die 20 on the topside 82
of the wafer 70, the process includes the step of forming a cavity
88 on a backside 90 of the wafer 70 (beneath the circuit on the die
20). This backside cavity 88 defines a thin layer 92 that includes
the circuit on the die 20. The backside cavity 88 will also define
the wafer support body 72 that is substantially thicker than the
thin layer 92 and the die 20. The thin layer 92 has a thickness
slightly greater than the die 20.
[0042] The cavity 88 on the backside 90 of the wafer 70 may be
formed using known semiconductor etching methods. In one
embodiment, the cavity 88 is formed using an anisotropic wet etch
using chemical solutions such as KOH, EDP or TMAH. A masking
material (not shown) such as silicon dioxide or silicon nitride may
be used for etching the cavity 88. The depth of the cavity 88 on
the backside 90 of the wafer 70 is application specific and will
depend on the desired thickness of the die 20. In one example,
where the desired thickness of the die 20 is to be about 15 .mu.m,
etching may be performed for sufficient time to define the thin
layer 92 to a thickness of about 22 .mu.m.
[0043] As shown in FIG. 6B, the next step is the formation of a
trench 76 around the circuit on the topside 82 of the wafer 70. As
mentioned above, the trench 76 will define the outer perimeter 74
of the die 20 having a circuit. The trench 76 may be formed using
known semiconductor etching methods. In one embodiment, the trench
76 is formed using an etch process such as reactive ion etching
(RIE), plasma etching or sputter etching. The depth of the trench
76 is application specific and will depend on the desired thickness
of the die 20 and the thickness of the thin layer 92. The trench 76
should have a depth of at least the desired thickness of the die 20
but smaller than the thickness of the thin layer 92 illustrated in
FIG. 6A. In the above example where the desired thickness of the
die 20 is to be about 15 .mu.m and the thin layer 92 is about 22
.mu.m, the trench 76 may be formed to about 18 .mu.m deep.
[0044] As shown in FIG. 6C, the process also includes a step of
forming tethers 78 on the topside 82 of the wafer 70. The tethers
78 also extend across and into select portions of the trench 76 and
between the wafer support body 72 and the die 20. The tethers 78
should be patterned. As described above, FIGS. 4 and 5 show top
views of some embodiments of patterned tethers 78, 178. Note that
FIG. 6C uses the reference number for the tethers 78 in FIG. 4.
However, the view shown in FIG. 6C would apply equally to the
tethers 178 shown in FIG. 5 and even for other geometric shapes of
tethers. What is critical is that the tethers form a bridge or
other connection between the support body 72 and the thin die 20 of
the wafer 70.
[0045] In one embodiment, the tethers 78 are made of a polyimide
material although other materials may be used such as other
thermoplastic materials or polymers. A polyimide material is
preferred because it can have a thickness ranging from a few
microns to tens of microns. Although the thickness of the tether
may be application specific, in one embodiment, a polyimide tether
may be between 2-10 .mu.m on the topside 82 of the wafer 70 and
5-30 .mu.m in the trench 76. The polyimide coating is preferably
applied to the wafer 70 using a spin coating process. Although a
spin coating process provides good uniformity and coating
qualities, other known application techniques could be used such as
spray, drop coating, and roller.
[0046] To perform the patterning, a photosensitive polyimide may be
used. Existing photosensitive polyimides permit the patterning of
relatively fine features. The patterning process may include spin
coating the polyimide and a drying step by hot plates or an oven.
In combination with a negative tone photo mask, the deposited
photosensitive polyimide layer may then be exposed to a standard I
or G lithography tool. The patterned polyimide tethers may then be
cured by conventional methods. Curing the polyimide film involves
the removal of the solvent carrier or other volatiles from the
polyimide layer and the hardening of the polymer into suitable
tethers.
[0047] If a photosensitive polyimide is not used, other methods of
patterning may be used such as conventional wet or dry etching
processes. A wet etching process will typically include that the
polyimide be patterned prior to final cure. A dry etching
processing may also include that the polyimide be patterned prior
to final cure.
[0048] As shown in FIG. 6D, the next step is to remove a thin layer
94 on the backside 90 of the wafer 70. The removed thin layer 94 is
shown in FIG. 6D as a dashed line below the thin die 20 and along
the widened cavity 88. The removal of the thin layer 94 exposes the
trench 76 but leaves the tethers 78 intact. In effect, the removal
of the thin layer 94 removes a portion of the thin layer 92 shown
in FIG. 6A to define a bottom surface 75 of the die 20. The thin
layer 94 may be removed using a variety of conventional etching
methods such as reactive ion etching (RIE), plasma etching or
sputter etching. The depth of the removed thin layer 94 is
application specific and will depend on the desired thickness of
the die 20, the thickness of the initial thin layer 92, and the
depth of the trench 76. As explained above, the removed thin layer
94 should have a depth sufficient to expose the trench 76 but not
as deep to remove the tethers 78. In the above example where the
desired thickness of the die 20 is to be about 15 .mu.m, the
initial thin layer 92 being about 22 .mu.m, and the trench 76 being
about 18 .mu.m, the removed thin layer 94 has a depth of about 7
.mu.m deep.
[0049] It can be seen in the figures that the thin semiconductor
die 20 is still attached to the surrounding support body 72 of the
wafer 70 by the tethers 78. The wafer 70 (having at least one die
20 and tethers 78) are now suitable for packing, shipping and
transporting to assembly plants where the thin dice may be
subsequent separated by breaking the tethers 78 by pick and place
operations. These further operations are explained in more detail
below. What has been described is a device and process that helps
in subsequent wafer separation of thin dice. The structure of the
wafer also makes it easier to ship and automate die pick and place
operations. This process also allows the surfaces of the die to be
maintained very clean prior to die attachment to other
surfaces.
[0050] The above figures illustrate a thin die that is
substantially square. It is noted that the present invention is not
limited to thin dice that are substantially square. While a square
die is illustrated, in some circumstances it may be desirable to
use other geometrical shapes for the die. Moreover, although the
procedures are described in the context of a silicon-based
semiconductor material, the present invention may also apply to the
formation of tethers on other types of semiconductor materials such
as gallium arsenide (GaAs). One of ordinary skill in the art with
the benefit of this application would realize that such other
geometrical shapes and semiconductor materials could be used.
[0051] Separation and Handling of a Thin Die
[0052] As described above, the thin die 20 is suspended on the
wafer 70 by thin tethers 78, 178 that can be made of a material
such as polyimide. As will be illustrated below, the tethers allow
a cohesive failure point that occurs along the outer perimeter 74
of the thin die 20 during subsequent pick and place operations. It
is preferred that the individual tethers 78, 178 be small to
minimize the amount of residual polyimide left on the area
extending on the die 20. The number of tethers 78, 178 around the
perimeter of the thin die 20 should be sufficient to ensure that
the die 20 does not fall off during wafer handling and shipping. As
described above, in one embodiment, the thin die 20 is attached to
the wafer by four (4) tethers 78, 178, one tether 78, 178 for each
side of the die 20.
[0053] An advantage of suspending a thin die 20 by tethers 78, 178
is that the die 20 is ready for pick and place operations without
any further processing steps at the wafer level. Additionally,
suspending the thin die 20 by use of tethers 78, 178 enables the
backside of the die 20 to be more easily shielded from
contaminants. As will be explained in more detail below, in one
embodiment for a pressure sensor, the backside of the die 20 is the
portion of the die 20 that is bonded or otherwise attached to a
pressure port. This backside surface needs to be clean from
contaminants for sensors.
[0054] A new process for separating and handling thin die is
illustrated in FIGS. 9A-9D and 10A-10D. FIGS. 9A-9D illustrate a
process to remove or otherwise separate a thin die 20 from a wafer
70. FIGS. 10A-10B illustrate a process of transporting the thin die
20 from the wafer 70 and placing the die 20 on a surface or
diaphragm 40. As illustrated in FIGS. 7 and 8, some of the tools
used to perform these processes are a backing 110 and a die handler
120.
[0055] FIG. 7 shows one embodiment of a suitable backing 110. The
backing 110 is preferably made of a metallic or other rigid
material such as aluminum. During general wafer handling and die
removal, the backing 110 is rigidly clamped, taped or otherwise
attached to the wafer 70. The backing 110 has an array or plurality
of holes 112. The holes 112 are spaced out to line up exactly with
the plurality of backside cavities 88 that are formed on the wafer
70 described above in FIG. 6A. The contours of the backing 110 are
shaped like standard wafer frames. This allows the backing 110 to
fit inside standard feeders and machine fixtures. The use of a
rigid backing 110 is important because the wafer 70 itself has very
little flexural strength due to the numerous dice surrounded by
square holes or trenches 76. The trenches 76 leave behind a thin
wafer skeleton that may be subject to fracture without the use of
the backing 110. Additionally, the trenches 76 have sharp comers
that act as stress concentrators. The purpose of the backing 110 is
to protect the wafer 70 against fracture during transporting and
handling as well as during die removal. The holes 112 in the
backing 110 allow an ejection pin (as shown in FIGS. 9A-9D) to move
freely in and out of every backside cavity 88 of the wafer 70.
[0056] FIG. 8 shows one embodiment of a die handler 120. The die
handler 120 is used for pick and place operations. Where the die 20
is used for a pressure sensor, the die handler 120 may also be used
to remove or separate the die 20 from a wafer 70 and install the
die 20 to a pressure port or diaphragm 40. In other applications,
the die handler 120 may be used to remove or separate the die from
the wafer and install the die to whatever other surface that the
die is to be mounted. The die handler 120 may be attached to an
automated machine to perform pick and place operations to each of
the plurality of dice 20 on the wafer 70.
[0057] In one embodiment, as shown in FIG. 8, the die handler 120
may have an upper body chamber 122, a rigid body portion 124, a
movable body portion 126, and a tip 128. The upper body chamber 122
is attached to the rigid body portion 124. As shown in FIG. 8, this
may be done by threading the upper body chamber 122 to the rigid
body portion 124. The upper body chamber 122 has a port 130 that is
configured for receiving a line to a vacuum source (not shown). The
upper body chamber 122 is preferably made of plastic but may be
made of other materials such as metallic materials.
[0058] The movable body portion 126 is movably attached to the
rigid body portion 124. In one embodiment, the movable body portion
126 has guide pins 132 that are capable of sliding within
cylindrical chambers 134 of the rigid body portion 124. The movable
body portion 126 is capable of moving up and down in relation to
the rigid body portion 124. Between the movable body portion 126
and the rigid body portion 124 is a piston 136. The piston 136 is
rigidly attached to the movable body portion 126 and movably
attached to the rigid body portion 124 within cylindrical chambers
138 and 140. The piston 136 has a ridge 142 that allows the piston
to be retained in the rigid body portion 124. A spring 144 is used
within a cylindrical chamber 140 in the rigid body portion 124 to
provide a compressive force to keep the piston 136 and movable body
portion 126 in the downward position. The rigid body portion 124,
the movable body portion 126, and the piston 136 are preferably
made of a metallic material such as aluminum, although other
materials may be used such as plastic.
[0059] The tip 128 is preferably made of a flexible material such
as rubber. The tip 128 is attached to the movably body portion 126.
As described and shown in FIG. 8, the spring 144 is configured to
allow the tip 128 of the handler 120 to also move in relation to
the rigid body portion 124.
[0060] As explained above, the port 130 in the upper body chamber
122 is configured for receiving a line to a vacuum source. A
passageway 146 is provided through the rigid body portion 124,
through the piston 136, through the movable body portion 126, and
through the tip 128. As will be explained below, this passageway
146 provides a vacuum suction force that will assist in pick and
place operations for the thin die 20.
[0061] Other configurations for a die handler 120 may be suitable
for the present invention. For example, the guide pins 132 may be
rigidly attached to the rigid body portion 124 and extend into
cylindrical chambers in the movable body portion 126.
Alternatively, the upper body chamber 122 may be removed and the
port 130 (attached to the vacuum source) may be directly connected
to the passageway 146 of the rigid body portion 124. In any event,
what is important is that the die handler 120 has some flexibility
when pressure is applied to the thin die 20 during pick and place
operations. Some of those features may include fabricating the tip
128 out of a flexible material such as rubber. Alternatively, the
die handler 120 could include a spring mechanism such as that
described in relation to FIG. 8.
[0062] What will now be explained is a procedure for pick and place
operations for separating and handling a thin die 20. Again, for
purposes of illustration, the description and figures are shown in
the context of the thin semiconductor die 20 described above in
FIG. 1. One of ordinary skill in the art with the benefit of this
disclosure will recognize, however, that the present invention
applies to other thin die applications.
[0063] Referring to FIGS. 9A-9D, a procedure for removing or
separating the thin die from the support body 72 of the wafer 70
will be explained. The thin die 20 is initially attached to the
support body 72 by an attachment mechanism such as that described
above having a plurality of tethers 78, 178. As illustrated in FIG.
9A, the wafer 70 (having a support body 72, at least one thin die
20, and tethers 78 (or 178)) is positioned on backing 110. One of
the plurality of holes 112 is positioned beneath the thin die 20.
The tip 128 of the die handler 120 is positioned above the thin die
20 on the wafer 70. Because no force is being exerted on the tip
128, the spring 144 within chamber 140 keeps the tip 128 in the
downward position by forcing the ridge 142 of the piston 136 to the
bottom of the cylindrical chamber 140 of the rigid body portion
124. The vacuum source connected to the port 130 is then activated
providing a vacuum to passageway 146. An ejection pin 150 is also
positioned in a spaced apart relationship beneath the thin die 20
and within a hole 112 of the backing 110.
[0064] Referring to FIG. 9B, the die handler 120 is then moved in
the downward direction (as shown by arrow A) toward the thin die
20. The tip 128 of the die handler 120 makes contact with the thin
die 20. The tip 128 of the die handler 120 continues in the
downward direction A to break the tethers 78 (or 178). The rigid
backing 110 holds the support body 72 of the wafer 70 in place.
This separates the thin die 20 from the support body 72 of the
wafer 70. The tip 128 of the die handler 120 continues in the
downward direction A until it makes contact with the ejection pin
150. This clamps the thin die 20 between the tip 128 of the handler
120 and the ejection pin 150. It is noted that when the tip 128 of
the die handler 120 travels in the downward direction A (to break
the tethers and make contact with the ejection pin 150), the piston
136 is permitted to move within the chamber 140 in an upward
direction (as shown by arrow B) to compress the spring 144. This
provides a soft landing of the tip 128 when it comes in contact
with the thin die 20 to prevent damage.
[0065] It is noted that the thin die 20 is detached from the wafer
70 by exerting a downward pressure. The application of a downward
force is an important feature of the present invention. An
alternative process such as pulling the thin die 20 up by relying
solely on the suction force exerted by the passageway 146 within
the tip 128 has proven to be unreliable. This is due to the fact
that a very small contact area of the die 20 limits the suction
force that can be exerted by the passageway 146 within the tip 128.
Relying solely on the suction force to detach the die 20 would
limit the tether design to being extremely weak. This would result
in requiring tight process controls on tether manufacturing and
would increase the risk of die separation during wafer handling and
shipping. In contrast, relying on a compressive force (against the
rigid backing 110) to break off the tether allows more flexibility
in varying the tether design. It also allows more tolerance in
variability in the tether fabrication process without compromising
the ability to separate the die 20 from the wafer 70.
[0066] In the preferred embodiment, the vacuum source remains
active through passageway 146 while the thin die 20 is detached
from the wafer 70. If the vacuum is turned off, the thin die 20 may
not be held horizontally during the breakage of the tethers. This
may cause the tethers to break at different times. If the tethers
do not break simultaneously, there is a risk that the last tether
will fold and act as a hinge, leaving the thin die 20 hanging by
one edge.
[0067] Referring to FIG. 9C, the tip 128 of the die handler 120 and
ejection pin 150 move together in the upward direction (as shown by
arrow C). This may further move the piston 136 within the chamber
140 in an upward direction (as shown by arrow D) to further
compress the spring 144. It is noted that during this step the thin
die 20 is preferably clamped between the tip 128 of the die handler
120 and the ejection pin 150. The clamped thin die 20 is then
extracted from the support body 72 of the wafer 70 by a
simultaneous upward motion in the upward direction C. Left
unclamped, the thin die 20 may be lost during extraction, as the
die 20 may come in contact with the residual tethers 78 (or 178)
left hanging around the perimeter of the support body 72 of the
wafer 70. Again, this is due to the fact that the net suction force
exerted by the vacuum source through the passageway 146 within the
tip 128 may not be strong enough to pull the thin chip 20 through
any residual tethers left hanging around the perimeter of the
support body 72 of the wafer 70.
[0068] Clamping the thin die 20 between the tip 128 and the
ejection pin 150 also eliminates the possibility that the die 20
will shift or rotate before or during extraction. Such shifts or
rotations could possibly cause the thin die 20 to collide with the
support body 72 of the wafer 70. Additionally, to minimize bending
or shearing, the ejection pin should have a diameter in close
proximity to that of the tip 128 and its upper surface should be
flat in relation to the thin die 20.
[0069] The die handler 120 and the ejection pin 150 may move
together in an upward direction C to extract the thin die 20.
Alternatively, having a spring 144 in the die handler 120, the die
handler 120 could be programmed to be stationary while the ejection
pin 150 provides the upward force. The spring 144 enables the
ejection pin 150 to provide the upward force by allowing the tip
128 of the die handler 120 to move upward with the ejection pin
150.
[0070] Referring to FIG. 9D, the die handler 120 may be moved in
the upward direction to lift the thin die 20 off the ejection pin
150. This will move the piston 136 within the chamber 140 of the
die handler 120 in a downward direction (as shown by arrow E) by
the compressive forces exerted by the spring 144. The ridge 142 of
the piston 136 will then rest in the bottom of the chamber 140.
With the vacuum source to the passageway 146 still active, the thin
die 20 remains on the tip 128 of the die handler 120. The ejection
pin 150 is now free to retract in it initial downward position.
[0071] As can be seen in the above-described separation and
extraction process, the use of a spring-mounted compliant pick up
head has several important advantages. First, the soft spring
limits the force when the tip 128 makes initial contact with the
thin die 20. Second, the soft spring limits the clamping force
exerted on the thin die 20 when the thin die 20 is clamped between
the tip 128 and the ejection pin 150. This reduces the risk of
damage to the thin die 20. Third, the spring eliminates the need to
synchronize the upward motions of the ejection pin 150 and the tip
128 of the die handler 120 as shown in FIG. 9C. If the tip 128 were
rigid, synchronizing these two moving parts while controlling the
clamping force would be difficult to achieve. Now, the tip 128 of
the die handler 120 can be programmed to be stationary while the
ejection pin 150 moves upward. Forth, the soft spring allows the
die handler 120 to be operated in displacement control, without any
need to monitor the clamping force. Finally, the soft spring
loosens the requirements on the precision and accuracy of the
stopping positions of both the tip 128 of the die handler 120 and
the ejection pin 150.
[0072] What will now be explained is a procedure for handling and
installing a thin die 20 on a diaphragm 40. As illustrated in FIGS.
10A and 10B, the thin die 20 (attached to the tip 128 of the die
handler 120) is moved from the support body 72 of the wafer 70 and
positioned above a diaphragm 40. It is noted that during this
handling procedure, the vacuum source attached to the passageway
146 within the tip 128 is active. As shown in FIG. 10C, the tip 128
of the die handler 120 is moved in the downward direction (as shown
by arrow F) to place the thin die 20 to the diaphragm 40 via a
coupling 50 (explained below). It is noted that when the thin die
20 and the tip 128 of the die handler 120 travels in the downward
direction F (and make contact with the diaphragm 40 and coupling
50), the piston 136 is permitted to move within the chamber 140 in
an upward direction (as shown by arrow G) to compress the spring
144. This provides a soft landing of the thin die 20 when it comes
in contact with the coupling 50 and the diaphragm 40 to prevent
damage.
[0073] As explained above, in the case of a thin die 20 for a
pressure sensor, a pressure-conductive coupling 50 is used between
the thin die 20 and the diaphragm 40. The coupling 50 may include a
corrosive-resistant pressure-conductive electrically insulating
material to conduct and transmit the sensed pressure from the
diaphragm to the thin semiconductor-sensing die 20. A suitable
coupling 50 is made of fused glass frit and screen-printed on the
diaphragm 40. Glass frit is useful because it electrically isolates
and prevents shorts from the metal diaphragm 40.
[0074] Referring to FIG. 10D, after the thin die 20 is placed on
the diaphragm 40 via coupling 50, the die handler 120 is moved in
the upward direction (as shown by arrow H). Prior to moving in the
upward direction H, the vacuum source to the passageway 146 within
the tip 128 is turned or switched off. This allows the tip 128 of
the die handler 120 to separate from the thin die 20. The die
handler 120 is now ready to perform its next pick and place
operation on a new die on the wafer 70.
[0075] Although FIGS. 10A-10D show the handling and installing of a
thin die 20 on a diaphragm 40, one of ordinary skill in the art
having the benefit of this disclosure would realize that the same
handling and installing steps may be taken to mount a thin die on
other surfaces for other applications.
[0076] What has been described is a new device and process for
separating and handling a thin die on a wafer. The present
invention permits the separation of a thin die handled and shipped
on the original wafer. The thin die can be separated or extracted
directly from the original wafer used to form the integrated
circuit on the die. Additional steps at the wafer level are avoided
before the pick and place operations.
[0077] The above description of the present invention is intended
to be exemplary only and is not intended to limit the scope of any
patent issuing from this application. For example, the present
discussion used a thin die for a sensor to describe the separation
and handling of a thin die. The present invention is also
applicable to separation and handling of other types of thin die
such as applications for smart cards, smart labels, actuators, and
multi-thin wafer designs. The present invention is intended to be
limited only by the scope and spirit of the following claims.
* * * * *